Lapping the CPU in a heatsink test platform is probably a controversial move that's bound to provoke reactions. Funny thing is, it was done a year ago, and photos of the CPU showing the copper top exposed by the lapping have been featured in many of our reviews. Yet, not a single comment. This article goes through the problems, investigations and explorations that led us to lap our Core i7-965 Extreme test CPU, and analyzes the results and implications.

Interesting investigation, even if I don't care enough to lap a CPU today. I guess enforcing lower manufacturing tolerances could be mean a significant rise in market price, it would be great if CPU manufacturers and heatsink makers could come together with tight specs for top of the line coolers and integrated heat spreaders. That is if Intel alone would be at all interested, which might be much to assume.

This is why I read hardware reviews on SPCR (and hardly anything else hardware-related on the net). I wish all published science was done with the same integrity, self-criticism and open mind as you guys repeatedly demonstrate in your computer equipment reviews. Thank you!

Testing must have taken a lot of patience. Furthering what MKK said, review sites won't have had the time or patience to get a conclusion like that, let alone users with limited resources. To repeat nicke2323's comment, this is why we need SPCR.

The results are interesting. If the heatspreader doesn't bow from the pressure of a heatsink mating surface, a convex heatsink seems superior for average (non-optimal) conditions. If some manufacturers consistently produce convex-ish heatsinks, they might have decided their flatness-tolerances weren't reliable and biased it to the safer convex shape. I say "safer" because a convex heatsink will have at least firm contact in the center of the heatspreader regardless of the heatspreader's shape.

This IHS deformation would likely explain why I never had great experience with a Noctua NH-D14 or aircooling in general. I had been watercooling for some time and there was a trend of convex bases starting around the time of the D-Tek FuZion waterblock and trending probably all the way until still now. At one point I decided to move back to air cooling after great reviews on the D14, but I had experienced poor temperatures and went back to water. Only after having moved to a new platform did I see good results on the heatsink I had shelved.

One thing I disagree with is this section

Quote:

Concavity of the IHS is a less critical factor with smaller CPUs, like the socket 1150, 1155 or 1156 CPUs. The dimensions of the IHS suface being smaller, the same degree of curvature leads to a smaller gap between IHS and heatsink base.

I would argue that it would be even more pronounced. When we first started de-lidding Ivy Bridge CPUs it immediately jumped out at me that the glue might be lifting the IHS too far from the core (link), and it was confirmed in Idontcare's extensive delidding thread.

If the concavity of the heatspreader can have such a huge effect on a soldered IHS like your 965, on one that is merely secured by TIM & silicone glue, like the more recent 1155 and 1150 CPUs, I feel it would be even more susceptible to deformation.

An Intel CPU is a flip-chip assembly, where one side of the die is mounted onto a PCB or ceramic substrate and the other is used for heat dissipation. For the large die CPUs (ie: not IVB or Haswell), Intel uses an Indium based solder as the TIM between the die and the IHS.

If heatsinks are deforming the IHS into (or further into) a concave shape, what's being equally deformed inside the package? The indium solder does deform over temp (that's part of the purpose - to be the middleman between the IHS's coefficient of thermal expansion and the die's)...but I can't imagine it would be visibly noticable/take the brunt of it. Seems to me the weakest link is the package substrate. Are these things bowing outward? If you look at a CPU with a formerly flat IHS and is now concave, is the backside (pin side) of the CPU equally convex?

Concavity of the IHS is a less critical factor with smaller CPUs, like the socket 1150, 1155 or 1156 CPUs. The dimensions of the IHS surface being smaller, the same degree of curvature leads to a smaller gap between IHS and heatsink base.

I would argue that it would be even more pronounced. When we first started de-lidding Ivy Bridge CPUs it immediately jumped out at me that the glue might be lifting the IHS too far from the core (link), and it was confirmed in Idontcare's extensive delidding thread.

If the concavity of the heatspreader can have such a huge effect on a soldered IHS like your 965, on one that is merely secured by TIM & silicone glue, like the more recent 1155 and 1150 CPUs, I feel it would be even more susceptible to deformation.

I think there are other issues with 1155 that complicate the matter -- the stock TIM for one. If the IHS is not making firm contact with the die, then a convex, high-pressure heatsink would bend it, and improve cooling, I agree. Hard for me to contribute much about cooling 1155 Ivy Bridge CPUs, as my experience is more limited. But I will say this about the Intel Core i5-2400 Sandy Bridge CPU on our small HS test platform: Its IHS is slightly convex.

CA_Steve wrote:

If heatsinks are deforming the IHS into (or further into) a concave shape, what's being equally deformed inside the package? The indium solder does deform over temp (that's part of the purpose - to be the middleman between the IHS's coefficient of thermal expansion and the die's)...but I can't imagine it would be visibly noticable/take the brunt of it. Seems to me the weakest link is the package substrate. Are these things bowing outward? If you look at a CPU with a formerly flat IHS and is now concave, is the backside (pin side) of the CPU equally convex?

This is a pertinent point, and I have to admit the last time I examined our i7-965E, I could not help notice that the corners of the PCB (substrate) are bowing upwards visibly. The subtrate, looked at from the IHS side, is concave! BUT -- you have to remember that the LGA1366 mechanism pulls the CPU down into the socket by the "lips" on two opposite edges of the IHS and locks it down pretty tightly. The IHS, which is stiffest around its perimeter, is pressed hard against the subtrate when the CPU is mounted, so I'm fairly certain the PCB actually gets straightened. Even if it doesn't, I doubt that PCB bowing has any impact on IHS/heatsink-base contact -- or on die/IHS contact. What impact it has on the longevity of our CPU I'd hate to think. (knocks on wood.) Ditto the 1155 CPU/socket btw. (see pic below of our i7-965E in the 1366 socket).

I've been paranoid about convex-base heatsinks for a good while - if motherboards can be bent, why not IHS? It's good to know the situation isn't THAT dire for your regular PC, but that the phenomenon still exists.

This was a good read and an excellent study into review methods and the mechanics involved. Again, as was pointed out, this is mostly pertinent to reviewers and anyone else who goes through a lot of repeat hardware installations, but it affects the regular/typical end users (opinions, product sales) through the review results.

I'm glad lapping improved the consistency in tests, but I'm certainly not doing it to my CPUs. Once was enough.

A very, VERY good article ! Thanks a lot for spending time on this issue.

MikeC wrote:

This is a pertinent point, and I have to admit the last time I examined our i7-965E, I could not help notice that the corners of the PCB (substrate) are bowing upwards visibly. The subtrate, looked at from the IHS side, is concave! BUT -- you have to remember that the LGA1366 mechanism pulls the CPU down into the socket by the "lips" on two opposite edges of the IHS and locks it down pretty tightly. The IHS, which is stiffest around its perimeter, is pressed hard against the subtrate when the CPU is mounted, so I'm fairly certain the PCB actually gets straightened. Even if it doesn't, I doubt that PCB bowing has any impact on IHS/heatsink-base contact -- or on die/IHS contact. What impact it has on the longevity of our CPU I'd hate to think. (knocks on wood.) Ditto the 1155 CPU/socket btw. (see pic below of our i7-965E in the 1366 socket).

When the hardware dies, it'll be time for a destructive test : take a cooler that can be tightened as much as you want, and tighten it until something breaks. It'll be interesting to see what gives first (I bet it will be the motherboard, the outer layers will get crushed).

As expected, there's quibbling about our lapping. This, from an ArsTechnica discussion:

ArsTechnicaForum wrote:

A: Normally I love their articles, but it really bugged me that they did not retest the same heatsinks after having the CPU lapped. I'm not quite willing to assume that all heatsinks with similar bases perform the same and use that as the basis for doing a pre/post comparison.

B: They did retest a bunch, but they weren't very clear when they switched over the reviews to the lapped CPU.......

A: They did? The article only listed the ETS-T40 as being retested... Maybe the other results are back in their original articles, but the beQuiet review wasn't updated with a postlap test, and that would have been the one that I most expected to be updated.

I wondered when a question about bequiet! would arise? To answer that question: bequiet! insisted on getting all the samples back for their techs to study, so we complied. It would have been interesting to retest them, but we just didn't have them any more. I would expect those samples -- and any HS with concave bases -- to do just about as badly as before.

We're not willing to go back and retest a bunch of heatsinks awarded Recommended or EC awards: We stand by those results, they are highly unlikely to test worse with the flatter CPU. Ditto the concave ones -- they will not test better. It's only the flat base HS with good secure mounting systems whose results are most likely change. There have been very few of these, and unfortunately, even fewer that we still have on hand.

Anyone asking for more retests should just read the article again - or refer to Mike's post here. The deformation issue affects flat base coolers, which I for one thought to be the "golden standard" coolers should aim for. The ones that caused the deformation won't obviously be greatly affected, and anyone with a dent in their goddamn base has a problem more serious than a nearly imperceptibly deformed IHS.

I have known about the IHS sag in my own i7 860 for years. I began to see the imprint of the IHS rim on the contact surface of every heatsink I tested. Since the heatsinks are copper under the nickle plating, they are fairly soft, so imprints always showed up. BTW--that is one way of detecting IHS sag on your CPU: the imprint it makes on the contact surface of the heatsink.

My suspicion is that this is a very common phenomenon, and that it should be factored into the performance of every heatsink. Certainly TR factored it into their heatsink designs. Perhaps parallel test rigs? One lapped, the other sagging? I for one, will continue to use my sagging IHS.

My suspicion is that this is a very common phenomenon, and that it should be factored into the performance of every heatsink. Certainly TR factored it into their heatsink designs. Perhaps parallel test rigs? One lapped, the other sagging? I for one, will continue to use my sagging IHS.

Two test rigs as you suggest would be good... but only if the power profile & thermal sensor of the 2 CPUs could be matched to... say 1W /1 degree. I'm not sure if this is feasible w/o a handful of the same model #s to try.

Back to the IHS issue, your word, "sag" probably describes it better than concave. The latter implies a curve, but it isn't necessarily a curve. The rim wall of the IHS tends to be slightly taller or positioned higher than the center, so under pressure, the center tends to sag a titch, while the rim wall does not compress at all.

At this point, it seems safe to say most high performance convex base coolers will provide the same performance as we achieve in our testing for >90% of users. Flat base ones should do about the same or only slightly worse (say a degree or 2) with a CPU that's new or never subject to a high pressure convex base HS. If the CPU is significantly concave, then a flat base HS could do a lot worse.

Concave base HS should always be avoided. In fact, we could save a lot of time & energy by simply rejecting all concave HS samples from our testing! Hmmmm....

If the concavity of the heatspreader can have such a huge effect on a soldered IHS like your 965, on one that is merely secured by TIM & silicone glue, like the more recent 1155 and 1150 CPUs, I feel it would be even more susceptible to deformation.

I think there are other issues with 1155 that complicate the matter -- the stock TIM for one. If the IHS is not making firm contact with the die, then a convex, high-pressure heatsink would bend it, and improve cooling, I agree. Hard for me to contribute much about cooling 1155 Ivy Bridge CPUs, as my experience is more limited. But I will say this about the Intel Core i5-2400 Sandy Bridge CPU on our small HS test platform: Its IHS is slightly convex.

MikeC, didn't you mean that the 1155 IHS is slightly concave?

I really enjoyed reading this article, it was exciting stuff!I might have misunderstood you, as English is not my first language, but you wrote this in the article:

MikeC wrote:

A year of testing after the lapping, our i7-965E CPU shows no sign of having become concave again. We're also not aware of any new records having been set for cooling in this past year, which might be indicated if the thinner IHS lead to improved cooling.

By saying that you are not aware of any new records having been set this past year, do you then mean that the heatsinks tested prior to a year from now performed better than those tested this last year? Because from reading your reviews I think it seems you are seeing better results. Of course whether this has to do with a lapped IHS is impossible to conclude.Cheers

But I will say this about the Intel Core i5-2400 Sandy Bridge CPU on our small HS test platform: Its IHS is slightly convex.

MikeC, didn't you mean that the 1155 IHS is slightly concave?

No, I meant convex. Otherwise I would not have laughed. It's quite slight, but there it is. Odd & somewhat unexpected. I'm not sure just how much it affects our test results -- compared to a flatter IHS. I suppose this is something we'll have to look at more closely some time soon. --sigh--

jAMBAZZ wrote:

I really enjoyed reading this article, it was exciting stuff!I might have misunderstood you, as English is not my first language, but you wrote this in the article:

MikeC wrote:

A year of testing after the lapping, our i7-965E CPU shows no sign of having become concave again. We're also not aware of any new records having been set for cooling in this past year, which might be indicated if the thinner IHS lead to improved cooling.

By saying that you are not aware of any new records having been set this past year, do you then mean that the heatsinks tested prior to a year from now performed better than those tested this last year? Because from reading your reviews I think it seems you are seeing better results. Of course whether this has to do with a lapped IHS is impossible to conclude.Cheers

The changed IHS did not lead to new lows in CPU temperature under load. The coolest results are mostly from HS tested before lapping. We did see quite good results from directtouch heatpipe base heatsinks, generally better than in the past, before lapping. Those are the only results that seem attributable to the flatter IHS.

I described what I see in my OHS as a sag because the rim is raised, and the center is elevated compared with the surface between the center and the rim. Think lunar crater with a flattened mountain in the middle.

And alignment really matters. Actually, check out this review for a better explanation. The bit about the IHS and pictures that show it are from "Looking for an Explanation" and afterward.

And alignment really matters. Actually, check out this review for a better explanation. The bit about the IHS and pictures that show it are from "Looking for an Explanation" and afterward.

Thanks for this!

I only had time to glimpse at the alignment pictures and conclusion, but the observations seem highly valid. Naturally it is again a very worn piece (by manufacturer standards), but I think it does present an argument in favour of flat bases - they would probably reduce the occurrence of the phenomenon, and could be mounted any which way for the best result. As everyone making flat bases is highly unlikely to happen, I guess we're back to finding out sub-IHS chip alignment (á la Arctic Silver instructions of yore) and spreading our TIM and aligning our heatsinks accordingly.

And alignment really matters. Actually, check out this review for a better explanation. The bit about the IHS and pictures that show it are from "Looking for an Explanation" and afterward.

Thanks for this!

I only had time to glimpse at the alignment pictures and conclusion, but the observations seem highly valid. Naturally it is again a very worn piece (by manufacturer standards), but I think it does present an argument in favour of flat bases - they would probably reduce the occurrence of the phenomenon, and could be mounted any which way for the best result. As everyone making flat bases is highly unlikely to happen, I guess we're back to finding out sub-IHS chip alignment (á la Arctic Silver instructions of yore) and spreading our TIM and aligning our heatsinks accordingly.

Do you have those old instructions, so you could quote the relevant part? Or link to it?

It seems that we are dealing not with simple convexity, but convexity that differs between one axis and the other -- two-dimensional variation in the third dimension (of convexity); the convexity is not axisymmetric. This suggests to me that when we look at the flatness of heatsink bottoms we will now have to gauge the convexity in both surface-wise dimensions.

Arctic Silver (not to be confused with Arctic Cooling, I believe) was the first and seemingly only manufacturer with such detailed instructions that paid attention to not only cleaning and dosage, but CPU chip orientation, prepping and/or tinting the IHS etc. Very helpful and informative for your amateur enthusiast.

Wonderful article, very nice and rewarding read. Real candy for all the DIY´ers amongst the readers, which probably are quite a few here, I for one.

Just a few thoughts:

1. Never buy anything else but a convex heatsink, if you want to play it safe. If there is a certain cooler you have fallen in love with, and you are unsure of the base surface, make sure you check it - and the IHS - carefully before buying/installing.

2.The old advice "Thou shalt never overtighten Bolts, Nuts and Screws" appilies in this case as well. It´s the most common cause of malfunction any mechanic will run into, and the subsequent damages are a source of income for him, that will never run dry. And when the article states, that the IHS hasn´t shown any sign of further deformation since lapping, it might suggest, that it could have been exposed to some degree of overtension at some point in it´s life.

3. Lapping is a work-around in some instances, not a kind of general remedy for improvement, that nescessarily will work in any instance, especially with a new IHS.

Apart from that, I wonder if it might be an idea to polish the IHS and the base of the cooler against each other with a rubbing compound, after the initial lapping with sandpaper? Somehow place the base in a vice, and use it as the grounding plane for the polishing of the IHS? That would shape the two to each other, like pistonrings and a cylinder slowly getting tighter during runnnig-in. And reveal in a very visible way, where the pressure points are.

Are you thinking of the way telescope makers grind their mirrors? If it works for them, why not cpu's/heatsinks?

Interesting idea. Wonder if it's practical?

Yeah, kind of like that. If two surfaces has to fit each other real tight, that´s the way to go.

Regarding practicality, it probably isn´t... Both parts would have to be fixated - unless you have a very steady hand - as you polish, which requires some thinking and tinkering, and when you´re finished, these two parts will be forever "married until death do them part".

But if you actually succeeded, I think it might give an extra degree or two. The ideal scenario is the two surfaces fitting so snugly together, that they don´t need no TIM. That´s impossible in the real world, where we pay for things with real money, though, but a polish like this might get us a little closer.

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